I am having a hard time understanding passive voice? Could someone please help?

Which sentence below is passive voice?
1. She served the customer the food.
2. The food was served to the customer.
3. The customer asked to change the order.
4. The server too the customer's order.

The subject is doing the action with an active voice verb. The subject is NOT acting with a passive voice verb.

1. Did SHE serve?

2. Did the FOOD serve?

3. Did the CUSTOMER ask?

4. Did the SERVER take?

Hint: Only one of these verbs is in the passive voice.